From the Guidelines
A response to TB M antigens indicates that a person's immune system has recognized and reacted to Mycobacterium tuberculosis antigens, suggesting exposure to tuberculosis bacteria. This response is typically measured through tests like the tuberculin skin test (TST) or interferon-gamma release assays (IGRAs) such as QuantiFERON-TB Gold or T-SPOT.TB 1. A positive response doesn't necessarily mean active TB disease; it could indicate latent TB infection (LTBI), where the person carries the bacteria but isn't sick or contagious. If someone tests positive for TB antigens, further evaluation with chest X-rays and sputum tests is needed to determine if they have active disease. For those with LTBI, preventive treatment with isoniazid (300mg daily for 6-9 months), rifampin (600mg daily for 4 months), or a combination of isoniazid and rifapentine (once weekly for 12 weeks) may be recommended to prevent progression to active TB 1. This immune response occurs because T-cells recognize specific proteins from the TB bacteria and release interferon-gamma, which is the basis for how these diagnostic tests work.
Some key points to consider:
- The tuberculin skin test has a specificity of approximately 99% in populations that have no other mycobacterial exposures or BCG vaccination, but the specificity decreases to 95% in populations where cross-reactivity with other mycobacteria is common 1.
- The likelihood that a positive test represents a true infection is influenced by the prevalence of infection with M. tuberculosis, and the predictive value of a positive tuberculin test is low in low-prevalence populations 1.
- Three cut points have been recommended for defining a positive tuberculin reaction, depending on the individual's risk of developing tuberculosis disease if they become infected with M. tuberculosis 1.
- IGRAs, such as QuantiFERON-TB Gold, have improved specificity compared to the tuberculin skin test and are less affected by BCG vaccination and infection with nontuberculous mycobacteria 1.
Overall, a response to TB M antigens is an important indicator of exposure to tuberculosis bacteria, and further evaluation and treatment may be necessary to prevent the development of active TB disease.
From the Research
Response to Tuberculosis (TB) Mycobacterial Antigens
A response to TB mycobacterial antigens indicates the presence of an immune response to Mycobacterium tuberculosis complex antigens. This can be detected through various tests, including:
- The Tuberculin Skin Test (TST) 2
- Interferon-Gamma Release Assays (IGRA), such as the QuantiFERON-TB Gold Plus (QFT-Plus) and the T-SPOT.TB 3, 2
- Antibody responses to specific Mycobacterium tuberculosis antigens, such as IgG and IgA responses to Mycobacterium tuberculosis chorismate mutase (TBCM), antigen 85B (Ag85B), early secreted antigen-6 (ESAT-6), and culture filtrate protein-10 (CFP-10) 4
Indications of a Response
A positive response to these tests can indicate:
- Latent Tuberculosis Infection (LTBI), which is a state of persistent immune response to Mycobacterium tuberculosis complex antigens without clinical, radiological, and microbiological signs of active disease 3, 5, 2
- Active Tuberculosis (TB) disease, although current tests have limited ability to differentiate between LTBI and active TB 6, 4, 5, 2
- A cellular immune response to M. tuberculosis, which can be useful for identifying individuals at risk of progressing to active TB disease 2
Limitations of Current Tests
Current tests for detecting a response to TB mycobacterial antigens have limitations, including:
- Limited sensitivity and specificity 6, 5, 2
- Inability to differentiate between LTBI and active TB 6, 4, 5, 2
- Potential for false-positive or false-negative results 3, 5, 2
- Need for improved understanding of the available tests to develop better tools for diagnosing LTBI and predicting progression to clinical active disease 5